Electron microscopy and mathematical analysis were used to determine the intensity of LPV oncornavirus production by a single cell in co-cultivated human diploid cells and cells of the continuous T-9 line. Maximum production of intracytoplasmic particles of A type was observed at 48 hours of cultivation, and extracellular virions at 96 hours. Mature virions of B type were more numerous than mature virions of C type in the mixed culture. On the whole the amolnt of mature virions was greater than that of immature ones, and the amount of intracytoplasmic A type particles was considerably greater than that of extracellular particles. By the total amount of production of all virus-specific structures and the two-wave pattern of virus production this mixed culture passaged at a ratio of HDC to T-9 cells 2000 : 1 resembled a culture of T-9 line. Thus, this study indicates that the infected HDC culture at later intervals of cultivation (96 hours) is a more active "producer" of LPV oncornavirus than the main line of T-9 cells or mixed HDC--T-9 culture.